The subject matter of the present invention relates to a firing system adapted for use in a perforating gun connected to a wireline conductor cable in a wellbore, and more particularly, to an exploding foil initiator (EFI) firing system for use in the perforating gun, the EFI firing system including an outer housing adapted to function as an electrical conductor for conducting a return current to ground potential from the EFI firing system and a wireline current from the wireline conductor cable.
Exploding foil initiators (EFI) have been used for initiating the detonation of a secondary explosive. For example, U.S. Pat. No. 4,788,913 to Stroud et al discloses a typical exploding foil initiator. In addition, U.S. Pat. No. 3,978,791 to Lemley et. al. and U.S. Pat. No. 4,471,697 to McCormick et. al. also disclose exploding foil initiator or "slapper" detonators. Furthermore, U.S. Pat. No. 4,441,427 to Barrett and U.S. Pat. No. 4,762,067 to Barker et al disclose the use of Exploding Foil Initiators in a perforating gun for propelling a flying plate into a secondary explosive and detonating the perforating gun. In addition, U.S. Pat. No. 5,088,413 to Huber et. al. discloses an exploding foil bubble activated initiator for use in a perforating gun, the Huber et. al. patent being incorporated by reference into this specification. However, although these initiators perform well, certain additional problems, associated with the use and/or performance of the EFI initiators in general and the exploding foil bubble activated initiator of the Huber et al. patent in particular, in a perforating gun wellbore environment, have yet to be solved.
For example, initiation of a perforating gun string in a wellbore can be accomplished using secondary explosives, such as HNS4. This explosive can be initiated using an EFI initiator that receives a high energy pulse from a fire set. Typically, the fire set consists of a high voltage power supply, an energy storage capacitor, and a switch that rapidly dumps stored energy into the EFI through a high frequency connector. This connector must have a very low effective series resistance (ESR). However, after detonation, the fire set must be contained in a protected housing which is isolated from the well fluids and the pressures in the wellbore. Therefore, a pressure bulkhead must be electrically and physically connected to the fire set and the EFI for electrically connecting the fire set and the EFI to ground potential so that the EFI can ultimately detonate the secondary explosives in the perforating gun string. In addition, when perforating oil wells, sometimes it is necessary to selectively shoot multiple guns in the same gun string. In order to detonate the gun selectively, the wireline voltage must pass through the upper guns in order to reach the lower guns in the gun string. Therefore, the pressure bulkhead which provides the EFI pulse must also provide a means to transfer the wireline voltage through the guns in the gun string. Typically, this is accomplished using a separate wireline feed through. When shooting perforating guns in a bottom up configuration, a detonating element must be placed on the bottom of the gun and the shaped charges are positioned above the detonating element in the perforating gun. This prevents a gun from detonating when the gun is partially flooded. A bottom-up configuration again requires that the wireline pass through the bulkhead of the EFI detonating element in order for the wireline to be connected to the bottom side of the detonating element. However, such a pressure bulkhead is very expensive to manufacture and is a short life part. In addition, the conventional bulkhead electrical property does not lend itself well to conducting a rapid high energy discharge pulse. Usually, the parameters of a bulkhead electrical property that suffer are the effective series resistance (ESR) and the effective series inductance (ESI). Since typical values of ESR and ESI are quite large, the energy storage capacitor inside the EFI must also be large. In addition, however, a wireline feedthrough for an EFI is difficult to fabricate for gun strings having small diameters.